In recent years, a UWB technique has attracted attention in an effort to further increase in speed of short-distance wireless communication in a wireless LAN or the like. The UWB is an abbreviation of a Ultra-Wide Band. The UWB technique is a wireless communication technique using a short-pulse RF signal. The UWB technique is characterized in that a frequency band occupied by a signal is ultra wide. At the present, a frequency is allocated in a microwave band under FCC in the USA, and the UWB technique is developed in an effort to realize the UWB technique around a several-gigahertz band. On the other hand, in research institutes, a millimetric-wave band UWB technique also begins to be examined to realize a gigabit-class wireless LAN. As methods of generating UWB signals, an IR (Impulse Radio) and a DS-SS (Direct Sequence Spread Spectrum) method are known for generating UWB signals.
FIG. 10 shows an example of a UWB signal generator based on the IR method (see IEICE Proceedings of the 2003 Electronics Society, p. 343). A UWB signal generator 101 in this example includes an optical pulse generator 102, an optical intensity modulator 103, a UTC-PD (uni-Traveling Carrier Photodiode) 104, and an antenna 105. An operation of the UWB signal generator will be described below. The optical pulse generator 102 outputs an optical pulse string having a half bandwidth of about 400 fs and output at a repetition frequency of 1 GHz. The optical intensity modulator 103 superposes 231-1 PRBS (Pseudo Random Bit Sequence) signals on the optical pulse string. The UTC-PD 104 converts the optical signal into an electric pulse signal having a half bandwidth of about 7 ps. The antenna 105 has a center frequency of about 120 GHz and an occupied band of about 49 GHz to release the electric pulse signal.
On the other hand, a receiver 106 includes an antenna 107, an SBD (Shottky Barrier Diode) 108, and an amplifier 109. An output from the receiver 106 is observed by an oscilloscope 110. The electric pulse signal released from the UWB signal generator is received by an antenna having the same characteristics as those of the above antenna. The SBD 108 envelope-detects the electric pulse signal received by the antenna. The amplifier 109 amplifies the electric pulse detected by the SBD 108. The oscilloscope observes a waveform of the electric pulse amplified by the amplifier. In this manner, data transmission at 1 Gbit/s can be achieved.
However, in this signal generator, since only an electric pulse matched with the characteristics of the antenna is output, the electric pulse cannot be transmitted simultaneously with another signal. Another disadvantage is that the signal generator must use pulse light. In the signal generator, since the output pulse mainly depends the characteristics of the antenna, a center frequency or a band is disadvantageously limited. In the signal generator, many pieces of light having frequencies which are not actually transmitted are wastefully generated.
In a UWB signal generator based on the DS-SS (Direct Sequence Spread Spectrum) method, a microwave/millimetric-wave UWB signal is generated by an optical heterodyne method. The UWB signal generator includes a signal source, two lasers, a phase lock loop, a Mach-Zehnder modulator, a bias power supply, a pulse pattern generator, a photodiode, and a spectrum analyzer. In the signal generator, the Mach-Zehnder modulator superposes carrier signals by phase modulation, the two lasers are synchronized in phase by the phase lock loop, and a UWB signal is generated by heterodyne extraction. In the signal generator, a stability control mechanism is disadvantageously complex and expensive.